Despite decades of research, characterization of the effects of polymer chain dispersity on the structural properties of block copolymer thin films remains challenging. We present an integrated experimental and modeling approach to characterize the morphology of thin films containing asymmetric diblock copolymers. Specifically, we used synergistic neutron reflectivity (NR) and self-consistent field theory (SCFT)-based modeling to realize unexpected morphology of thin films containing asymmetric copolymers. Using NR, a highly stable and reproducible mixed phase of coexisting cylinders and lamellar domains was discovered in asymmetric poly(deuterated-styrene-b-n butyl methacrylate) (dPS-PBMA) copolymer thin films containing 34% volume fraction of dPS. SCFT reveals how to obtain such a thermodynamically stable morphology in the presence of disperse majority block and asymmetric interactions of polymer species with surfaces. Stabilization of the coexisting domains is a consequence of the depth segregation based on chain-length distribution. The asymmetric chains microphase-separate into cylindrical domains close to the substrate and near-symmetric chains form lamellar domains at the air interface. In the absence of dispersity, the coexistence of cylindrical and lamellar domains is thermodynamically unstable because of the absence of depth segregation. Overall, such an effect of dispersity on diblock copolymer thin-film morphology reveals a unique and powerful strategy to create coexisting nanoscale domains and tailor properties of thin films.